1. Field of the Invention
The present invention relates to an access unit for a local area network (LAN) and, more particularly, to such an access unit used for connecting a plurality of workstations by branching one connector in a concentrator to provide a switching operation to add the workstations in the local area network through a loop topology.
2. Description of the Background Art
In a conventional local area network with loop topology, a concentrator is used for providing a switching operation to add a plurality of workstations. Each of the plurality of workstations are wired from the concentrator provided in a wiring room, so that the plurality of workstations are wired loop-like via the concentrator.
However, the number of connectors in the concentrator for connecting the workstations is limited in the above-described local area network. It is difficult to add workstations beyond the number of the connectors. It is, therefore, difficult to expand the network.
Conventionally, to meet with a request of expansion, an access unit as shown in FIG. 4 is used. An access unit (a) of FIG. 4 is a sort of branching device connected to one connector of a concentrator (b). Through the access unit (a), a plurality (three, in this example) of workstations ws.sub.1 to ws.sub.3 are connected. Using the access unit (a), it is possible to connect one cable (c) only to the concentrator (b) even when a plurality of workstations ws.sub.1 to ws.sub.3 are provided. It is not necessary to individually wire the workstations ws.sub.1 to ws.sub.3 to the concentrator (b). It is possible to expand the network.
Meanwhile, to construct a local area network with loop topology by connecting the workstations ws.sub.1 to ws.sub.3, the access unit (a) and the concentrator (b), to each other as shown in FIG. 4, a relay in the access unit (a) must be turned on when each of the workstations ws.sub.1 to ws.sub.3 are connected to each connector of the access unit (a) and a relay in the concentrator (b) must be turned on when the access unit (a) is connected to one connector in the concentrator (b). Thus, the relays must be turned on at the two stages from the view of the workstations ws.sub.1 to ws.sub.3 connected to the access unit (a).
Therefore, conventionally, the relay in the concentrator (b) and the relay in the access unit (a) are individually operated. See, for example, Japanese Patent Unexamined Publication No. 63-284945. That is, the relay of the concentrator (b) is operated in response to direct current from the workstations ws.sub.4 to ws.sub.5, or the access unit (a), while the relay of the access unit (a) is operated by power from a power line (l) specific to workstations ws.sub.1 to ws.sub.3, in addition to the cable (c) for transferring the data.
However, for the above construction, the additional power line (l) is needed to connect the access unit (a) to the workstations ws.sub.1 to ws.sub.3. The wiring becomes complex unlike the cable construction of connecting the access unit (a) and the workstations ws.sub.4 and ws.sub.5 to the concentrator (b). Basically, the normal standard data connector cannot be used, which is very inconvenient.
On the contrary, it may be proposed that as the output signals from the workstations, a DC signal is superposed as an offset on an AC signal which transfers the data, so that both relays are operated by the DC signal.
In the case where the DC signal outputted from the workstations operates both relays, an input resistance from the side of the workstations is different than the case in which the DC signal outputted from one workstation is forwarded to the subsequent-stage workstation and the case in which the DC signal outputted from one workstation is forwarded to the concentrator, so that the voltage level of the DC signal is varied.
Further, since each workstation detects the breaking of the connecting wire and the short circuit by detecting the voltage level of the DC signal outputted therefrom the workstation may determine the occurrence of a problem even when actual trouble does not occur if the voltage level of the DC signal is varied by turning the relays on or off which is inconvenient.